1. Field of the Invention
The present invention relates to clinical eye tracking systems, and more particularly to a self-contained, portable, high speed, video oculography goggle based system utilizing region of interest digital image processing.
2. Background Information
Accurate eye position recording and monitoring in three dimensions (3D-yaw, pitch and torsion rotation about line of sight) is a significant clinical diagnostic tool in the field of vestibular disorders such as vertigo and other neurological disorders. A non-invasive technique for recording eye position relative to the head is to use a camera to record eye position relative to the head, known as video oculography or VOG. VOG systems are used by Vestibular Researchers, Ophthalmologist, Otolaryngologists, Physical Therapists, Neurologists, Audiologists, Balance Clinicians, Neurophysiologists, Physiologists, Neuroscientists, Occupational Therapists, and others.
Image processing software is utilized to interpret the images to provide objective data of eye position. This type of image processing software is described in “A GEOMETRIC BASIS FOR MEASUREMENT OF THREE-DIMENSIONAL EYE POSITION USING IMAGE PROCESSING” Vision Res. Volume 36. No. 3, Moore et al., pp 445-459, 1996, which is incorporated herein by reference.
The existing VOG systems can be categorized as either earth mounted or head mounted systems. The oldest method uses earth fixed cameras and attempt to limit movement of the head. The relative movement of the head and the camera would be interpreted as eye movement. These systems attempt to stabilize or immobilize the head with head holders, head rests, chin rests, or bite bars. Although archaic, this type of system is still used extensively in some laboratories and many clinical environments. The biggest disadvantage of these systems is the inability to remove all head movement. Even the smallest head movements (e.g. resulting from breathing, talking, involuntary postural modification, and from fatigue etc.) cause significant inaccuracy in the measured eye movement. These systems are particularly unsuitable when inertial stimuli (e.g. a rotational chair) are delivered to a subject in order to produce vestibular responses, since these stimuli also tend to generate head movement. This equipment is often heavy and bulky since it must be strong enough to support and attempt to restrain the head of a subject.
Another classification of earth mounted VOG systems are systems that attempt to measure eye movement using a space fixed (earth mounted) camera without a head holder mechanism. In general, these systems attempt to deal with head movement by first tracking the head and then the eye within the head. In practice, a subject must actively suppress their head movements to within a small range of translations in order to stay within view of the camera. Further, rotations of the head are quite difficult to detect using image processing and so these systems suffer from an inability to distinguish between a change of eye position in the head or a change of head position during maintained gaze. These systems must also use a wide-angle lens in order to digitize an image that includes the head movements. Consequently, little picture resolution is available for the analysis of the eye position. As a result of these limitations, these systems are generally only able to measure horizontal and vertical changes in relative eye/head position
Another earth mounted VOG system attempts to measure the eye position by first tracking the position of the head and then moving a camera or mirrors to get an image of the eye with higher magnification and resolution. These systems also share many of the disadvantages of the other VOG earth fixed camera systems including the inability to accurately distinguish between head translation and rotation. Further, the mechanisms used can be complicated expensive noisy and distracting.
A second classification of VOG systems is the head mounted system. In one type of head mounted VOG system, head mounted cameras are supported by an adjustable headband often modified from the helmet insert taken from a mining or welding helmet. The cameras may be mounted above the eyes and are directed down towards hot mirrors that reflect an infrared image of the eye. Head mounted eye movement recording systems are less prone to the errors from head movement, because the cameras move with head. Further this method for attaching the cameras to the head is particularly popular because the headsets can be easily fitted to any subject without modification. The camera mounting position above the eyes also seems fairly natural because hardware tends to stick up into the air. This placement keeps the centre of gravity closer to the head and reduces the inertial lag on yaw head movements. Despite these advantages, all head mounted video eye movement measurement systems obviously suffer from the need to wear equipment and be connected, via leads, to the analysis hardware. Further, the headband can be painful if it is tightened enough to effectively suppress slippage of the headset during head movement.
The camera may also be mounted to the side of the headband head mounted VOG systems. The main advantage of mounting cameras to the side rather than above the eyes is that the centre of mass of the headset tend to be further back towards the head and so these headsets don't tends to pitch the subject's head forward as much as some other arrangements. This camera position also can provide better power supply and data output access (i.e. the electrical and control feeds). The main disadvantage of this mounting position is that the headsets tend to become quite wide. These headsets tend to move relative to the head during the yaw head movements that are common during vestibular testing.
The camera may also be mounted in front of headband in the headband head mounted VOG systems. The main advantage from mounting cameras towards the front of the subject is that no hot mirrors are required to reflect an image of the eye into the cameras. This lack of hot mirrors simplifies the construction and adjustment of the headsets and may improve the quality of video images. However, while front mounted cameras might suit light occluded systems where darkness prevents the subject from seeing them, they don't suit most video headsets that use headbands because these may not have an open field of view. Apart from the obstruction to vision, cameras in front of the subject can provide visual suppression and orientation cues that may affect their eye movement responses. The headsets with front mounted cameras also tend to have a centre of gravity that is further away from the head.
In place of the headband, some head mounted VOG systems utilize goggles, similar to those on diving masks, in order to attach cameras to the subjects face. These headsets benefit from a silicon skirt that conforms to the face and stabilizes the cameras. Goggles also leave the head clear for the use of other devices that may be utilized in various clinical applications. Another advantage of goggles is that they are well suited for the construction of light occluding headsets as well as those with an open field of view, or those that are convertible between the two. The disadvantage of goggles style head mounted VOG systems is that they can be uncomfortable if the cameras and other hardware is too heavy and weighs down on the subject's head.
Some research head mounted VOG systems use video cameras mounted on the headset with individually molded plastic or fiberglass masks. These masks are particularly stable and good at suppressing relative camera and head movement. Molded masks also tend to spread the weight of the video headset over a large surface area and do not produce the pressure points characteristic of some other methods. However, individually molded masks can be time-consuming and costly to make and are therefore not convenient for the clinical testing of large numbers of patients. Hybrid masks that combine a headband and standard molded mask section do not have these disadvantages but do not seem to benefit from the advantages either.
Another head mounted VOG system utilizes a helmet for camera mounting. The helmet style video headset benefits from a more even distribution of weight over the top of the head and from the balance provided by more weight towards the back. Helmet style video headsets are heavier than many other systems and so they tend to shift around during vigorous head movement. They are also quite bulky and prevent the application of head holders.
Another head mounted VOG system utilizes standard glasses construction (i.e. spectacle) for camera mounting. The advantages of spectacle type video headsets include that they can be very small and light, and are easily transportable. The disadvantages of this method include the discomfort from heavy equipment resting on the bridge of the nose. With very small contact area, spectacles can also be prone to movement relative to the head in response to inertial forces.
There remains a need for truly portable VOG systems. Further, there continues to be a need for accurate meaningful output for the clinicians in VOG systems without significant discomfort to the patients.
The above discussion concentrates on the deficiencies in the mechanical design of existing VOG systems. In addition to those issues, existing VOG systems are designed as one-of-a kind testing structures. This approach leads to expensive end products. Existing VOG systems also suffer from poor camera design, camera power supply issues, and data transfer problems.
Analog cameras in existing VOG systems provide data regarding eye position for analysis as is known in the art. During testing the visual image of the eye(s) is often displayed in real time as a method for the clinician to follow and interpret the data. In other words a real video image of the patient is displayed with a graphed display of the data (e.g. a chart of eye vertical and horizontal position change over time). These may also be recorded for later review. The realistic eye image of the video does not always easily illustrate eye movement.
Clinicians have stated that existing VOG systems on the market suffer from the following drawbacks: the excessive weight of goggles, they can't be used with droopy eyelids; difficulty with set-up; effective torsion measurements of the eyes are not available; lack of the sensor for head positioning; difficulty in viewing eyes; limited in the number of targets presentable to the patient; low sampling rates; software limitations and inflexibility; no ability to focus the camera; and concerns over image resolution.
There is a need to address at least some of these problems as well and still provide a portable, affordable VOG system providing accurate meaningful output for the clinicians in VOG systems.